1. Field of the Invention
This invention relates to a process for improving the thermal stability of jet fuels which have been sweetened by oxidation, for example, by the Bender and Merox processes.
There are very stringent specifications for kerosines used as jet or aviation turbine fuels. In addition to having the correct hydrocarbon composition, the kerosine must contain less than 0.003% by weight of mercaptan and exhibit satisfactory thermal stability.
Higher boiling hydrocarbon fractions, particularly kerosine and jet fuels, are generally sweetened by oxidation using a variation of the Bender or Merox process. A fixed bed Merox process is most commonly employed. The term fixed bed refers to the fact that the catalyst for the Merox process is impregnated or fixed onto a bed of catalyst support material, such as activated charcoal. The catalyst, in the presence of alkali and oxygen, promotes the oxidation of mercaptans present in the fuel to disulfides according to the equation: EQU 4RSH+O.sub.2 .fwdarw.2RSSR+H.sub.2 O
A similar reaction occurs in other processes where sweetening is effected by oxidation. The term sweetening refers to the conversion of mercaptans to disulfides and the elimination of the offensive mercaptan odor. The disulfides are oil-soluble and remain dissolved in the jet fuel.
Certain distillates, after conventional sweetening by oxidation, and even after further purification by treatment with clay, fail the Jet Fuel Thermal Oxidation Tester (hereinafter JFTOT test) and are unsuitable for use as jet fuels.